For a number of years, traffic control systems for controlling vehicle movement through intersections have been controlled, to various extents, by detectors which detect the number of vehicles entering the vicinity of the intersection. At first, these detectors were pressure treadles and other devices mechanically actuated by a passing vehicle. Such detectors were susceptible to deterioration and climate variations; therefore, other detectors were developed which did not function on mechanical principles. These involved radar sensors magnetic pick-ups, sonar units and induction loop pick-ups. Of these, one of the most prominently used mechanisms was the induction loop controlled detector. In this device, a loop is embedded in a roadway adjacent the intersection and the inductance of the loop is affected by vehicles entering and leaving the field of influence of the loop. In initial systems, the loop was connected to an oscillator. Changes in the frequency, and/or amplitude of the oscillator output were analyzed by analog circuits for indicating a vehicle detection. Although extensively used, this type of analog system was not extremely sensitive and varied substantially with ambient conditions. In an attempt to diminish the effect of ambient conditions, some systems were provided with a fixed frequency oscillator which was beat with the loop oscillator to produce an analog signal representative of the presence of metal within the field of influence of the detecting loop. These systems did not prove extremely satisfactory because the ambient conditions still affected the operation of the loop, irrespective of the presence of metal in its field of influence. Thus, these systems often involved a feedback tuning arrangement for the fixed frequency oscillator which attempted to bring the loop oscillator and reference oscillator into a controlled relationship preparatory to detection. The feedback tuning arrangements for the fixed frequency oscillator presented difficulties which were compounded by the analog nature of the total detector systems.
These disadvantages of prior attempts to use a loop detector were overcome by a digital detecting arrangement disclosed in U.S. Pat. No. 3,868,626. In this prior patent, two oscillators are employed. One oscillator has a fixed frequency and the other oscillator has a frequency varied according to the condition of the detecting loop. This type of system creates a pulse count during a counting interval. Detection can be recorded either by varying the counting interval by the loop controlled oscillator or by varying the rate of count by the loop controlled oscillator. In either of these arrangements, when the pulse count of a given counting interval differs from a reference count, by a threshold number of magnitude, a detection is indicated. These systems have been quite successful in overcoming the disadvantages of prior systems for using a loop to detect the presence of a vehicle adjacent an intersection. The oscillators do not require periodic tuning with respect to each other during continued operation of the detector. The detection is positive and the sensitivity of the detector system can be controlled by changing the threshold number or magnitude.
The present invention relates to an improvement in the digital detector shown in U.S. Pat. No. 3,868,626, which improvement involves a new system for comparing the pulse count to a reference count. This new system produces a coded signal output representative of the difference between the pulse count and the reference count. In addition, the sensitivity of the new system is substantially improved by employing a means for reading the coded signals and determining whether or not they exceed a magnitude which is adjusted to change the sensitivity of the loop detector. In this manner, the sensitivity may be varied over a wide range without being affected by the parameters of the loop and any interference with the loop detecting field.